That's the question the US Geological Survey set out to answer after a spate of tremors in the Midwest—an area not usually known for earthquakes—alerted scientists to the possibility that some of them might be man-made.

Seismic activity in the Midwest started increasing around 12 years ago but picked up significantly in the past few years, says seismologist Bill Ellsworth, the lead author of a new USGS study examining potential links between fracking and earthquakes in the region. Since 1970, the baseline for earthquakes in the Midwest measuring above a 3.0 hovered at around 21 per year, but beginning in 2001, that number began to rise. There's been a "remarkable increase" in the past few years: The number of 3.0-plus earthquakes rose from 29 in 2008 to 50 in 2009, then to 87 in 2010, and in 2011 to a staggering 134. Something unusual was going on, but what? As Ellsworth and his colleagues at USGS ask in the study, "Is this increase natural or manmade?" And if it's man-made, is fracking—which has ramped up in the region in the past several years—to blame?